This invention relates to a printed circuit board for transmission of electrical signals at high frequencies, which has a small coefficient of thermal deformation, including thermal expansion and contraction.
There is a conventional printed circuit board commercially available having a small coefficient of thermal deformation which is formed by pasting copper foil on the outer surface of an insulating layer of fiberglass fabric containing polytetrafluoroethylene (PTFE). However, this conventional printed circuit board has a dielectric factor which can be as large as about 0.002 at 10 GHz, and is not often used as a printed circuit board for high frequencies.
A printed circuit board having a high specific permittivity has been proposed, which is formed by pasting copper foil on the outer surface of an insulating layer of polytetrafluoroethylene in which a filler consisting of titanium oxide powder or alumina (aluminum oxide powder) is mixed. However, this printed circuit board has a thermal expansion coefficient with respect to its surface direction and thickness direction of as much as 1.times.10.sup.-4 /.degree. C. Obviously, it is thermally unstable and cannot preferably be used.
Therefore, it is an object of the present invention to provide a printed circuit board which substantially eliminates the above-mentioned faults of prior art printed circuit boards and eliminates a great increase in the expansion coefficient, even if a filler is mixed therein for the purpose of increasing the specific permittivity to a high level, and which has excellent properties in a high-frequency transmission region.